As used herein, the term ‘dynamic range’ (DR) may relate to a capability of the human psychovisual system (HVS) to perceive a range of intensity (e.g., luminance, luma) in an image, e.g., from darkest darks to brightest brights. In this sense, DR relates to a ‘scene-referred’ intensity. DR may also relate to the ability of a display device to adequately or approximately render an intensity range of a particular breadth. In this sense, DR relates to a ‘display-referred’ intensity. Unless a particular sense is explicitly specified to have particular significance at any point in the description herein, it should be inferred that the term may be used in either sense, e.g. interchangeably.
As used herein, the term high dynamic range (HDR) relates to a DR breadth that spans the some 14-15 orders of magnitude of the human visual system (HVS). For example, well adapted humans with essentially normal vision (e.g., in one or more of a statistical, biometric or opthalmological sense) have an intensity range that spans about 15 orders of magnitude. Adapted humans may perceive dim light sources of as few as a mere handful of photons. Yet, these same humans may perceive the near painfully brilliant intensity of the noonday sun in desert, sea or snow (or even glance into the sun, however briefly to prevent damage). This span though is available to ‘adapted’ humans, e.g., those whose HVS has a time period in which to reset and adjust.
Until fairly recently, displays have had a significantly narrower DR than HDR. Television (TV) and computer monitor apparatus that use typical cathode ray tube (CRT), liquid crystal display (LCD) with constant fluorescent white back lighting or plasma screen technology may be constrained in their DR rendering capability to approximately three orders of magnitude. Such conventional displays thus typify a low dynamic range (LDR), also referred to as a standard dynamic range (SDR), in relation to HDR.
To support backwards compatibility as well as new HDR display technologies, an HDR image may be represented by a tone-mapped image (typically referred to as the baseline image) with additional metadata comprising grayscale luminance ratios (typically, referred to as a ratio image). On one hand, the tone-mapped image may be used to provide support for standard dynamic range imaging workflows (e.g., displaying images on SDR imaging devices). On the other hand, the additional metadata (e.g., the ratio image) may be used with the tone-mapped image to generate, recover, or present an HDR image (e.g., by an HDR display).
As used herein, the terms “up-sampling” or “up-scaling” relate to the process of transforming one or more color components of an image from one spatial resolution to a second, higher, spatial resolution. For example, an image may be up-sampled from a 720×480 spatial resolution to a 1280×720 spatial resolution.
As used herein, the terms “down-sampling” or “down-scaling” relate to the process of transforming one or more color components of an image from one spatial resolution to a second, lower, spatial resolution. For example, an image may be down-sampled from a 1280×720 spatial resolution to a 720×480 spatial resolution.
As used herein, the term “windowing” relates to the process of selecting and displaying an area of the image that is smaller than the total area of the image. For example, if the original image has a spatial resolution of 1280×720 pixels, a window operation may select to display only portion of that image, e.g., an area of 720×480 pixels with origin the top, left-most pixel, of the image.
Given a coded image comprising multiple coded regions, the terms “partial decoding” or “decoding in part” relate to a decoding process where only part of the coded image is decoded, either in the spatial domain or a transform domain. For example, given a JPEG image which is coded using a plurality of 8×8 blocks of DCT (Discrete Cosine Transform) coefficients, partial decoding in the transform domain may be performed by decoding each coded block using an N×N inverse DCT, where N is an integer and 1≦N<8. Similarly, partial decoding in the spatial domain may be performed by decoding only a sub-set of the total number of 8×8 blocks in the coded image.
Mobile devices, such as smart phones or electronics tablets, have typical screen resolutions that are much smaller than the native resolution of captured SDR or HDR images. In most cases, before displaying a high-resolution image on such a device, after decoding, the image is post-processed by such operations as windowing, scaling, and cropping. Under existing techniques, these operations may require extensive memory and computational resources.
The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, it should not be assumed that any of the approaches described in this section qualify as prior art merely by virtue of their inclusion in this section. Similarly, issues identified with respect to one or more approaches should not assume to have been recognized in any prior art on the basis of this section, unless otherwise indicated.